The present invention pertains generally to labyrinth seals of the type used in steam turbines and, more particularly, to such seals wherein the clearance between the seal and the shaft may be controlled, adjusted, or varied in response to changes in the operating conditions of the turbine.
Non-contacting packing ring labyrinth seals are commonly used in steam turbines to prevent excessive steam leakage between regions of differential pressure. Such seals conventionally comprise a series (generally five or six) of packing or sealing rings disposed in annular grooves in the turbine casing. These rings typically include a plurality of spaced-apart annular teeth extending radially inward from the turbine casing to within close proximity of the rotating shaft surface, with the amount of the clearance determining the rate of leakage. The effectiveness of these seals is a major factor in determining the efficiency of the turbine, since leakage of steam through the seals results in a loss of work-producing energy in the system.
It is common practice, during start-up, to preheat the high pressure (HP) section of a turbine by pressurizing it with steam while the shaft is being rotated by turning gear. In a combined high pressure and reheat opposed flow turbine section, excessive steam leakage past the intersectional packing seal may lead to undesirable "turning gear roll-off" and damage to the system. Control of the clearances between the rings and the shaft is, also important for the safe operation of the turbine system.
Maintenance of proper clearance is complicated by vibration of the shaft and distortion of stationary components, particularly during periods when it is accelerating or decelerating through its critical speeds as well as thermal transients. These events occur during start-up and shut-down and during major load changes. Thus, during steady state operation of the turbine, such as occurs under significant load, relatively smaller clearance is desired. Relatively larger clearance is required, however, during occurrence of transients, so as to avoid rubbing and damage to the seals and shaft.
Prior attempts to satisfy these two conflicting requirements involved provision for the radial displacement of the segmented sealing rings between a larger diameter position, where a relatively larger clearance accommodated shaft transients, and a smaller diameter position, where a relatively smaller clearance minimized leakage around the steady shaft. Such a system is disclosed, for example, in U.S. Pat. No. 4,436,311 wherein springs bias the segmented sealing rings radially outwardly, the basing force being overcome by steam pressures within the turbine as it comes up to steady-state operation. As will be described more fully hereinafter, this system suffers in that each of the rings in the series is acted upon by a different force, which force is partly determined by the position or condition of the upstream rings, such that closure of each ring in the series prevents or delays closure of succeeding downstream rings. This system also fails to solve the problem of steam leakage control during prewarming.
A second such prior art system, disclosed in a paper entitled Clearance Control System of Packing Rings for Steam Turbine by E. Tsunoda, M. Kikuchi and T. Ekeda at the Seminar on FOSSIL PLANT RETROFITS FOR IMPROVED HEAT RATE AND AVAILABILITY, Dec. 1-3, 1987, at San Diego Calif., utilized bellows to facilitate the radially inward displacement of the packing ring segments, the bellows being operated by the admission thereinto of operating steam, each bellows being operated upon by steam passing upstream of its own packing rings. For reasons which will be hereinafter explained, this system suffers from the same shortcoming as that previously described. In addition, it is believed that reliability problems inherent in the use of bellows in the hostile environment of a steam turbine makes attractive an alternative.